2 * Mach Operating System
3 * Copyright (c) 1992, 1991 Carnegie Mellon University
6 * Permission to use, copy, modify and distribute this software and its
7 * documentation is hereby granted, provided that both the copyright
8 * notice and this permission notice appear in all copies of the
9 * software, derivative works or modified versions, and any portions
10 * thereof, and that both notices appear in supporting documentation.
12 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
13 * CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
14 * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
16 * Carnegie Mellon requests users of this software to return to
18 * Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU
19 * School of Computer Science
20 * Carnegie Mellon University
21 * Pittsburgh PA 15213-3890
23 * any improvements or extensions that they make and grant Carnegie Mellon
24 * the rights to redistribute these changes.
26 * from: Mach, Revision 2.2 92/04/04 11:33:55 rpd
27 * $FreeBSD: src/sys/i386/boot/biosboot/README.MACH,v 1.7 1999/08/28 00:43:10 peter Exp $
28 * $DragonFly: src/sys/platform/pc32/boot/biosboot/Attic/README.MACH,v 1.2 2003/06/17 04:28:34 dillon Exp $
31 ********NOTE: This is not all relevant to the 386BSD version**********
33 AT386 Protected Mode Bootstrap Loader
34 =====================================
36 1. Overview of Startup
39 After the system is rebooted, the BIOS bootstrap routine reads Sector
40 1, Track 0 into memory at location 0000:7C00H. If location 0000:7DFEH
41 (last two bytes of that sector) contains the value AA55H, the BIOS
42 bootstrap routine will transfer control to 0000:7C00H. Otherwise, the
43 boot code in that sector is bad and the boot routine stops.
45 For DOS compatibility reason, one extra stage of boot is required if
46 the boot device is a hard disk. The first sector of the hard disk will
47 contain the MOS-DOS boot code and a boot record partition table.
48 When this sector is loaded into 0000:7C00H, it will relocate itself
49 to somewhere else and then load the first sector of the active
50 partition into 0000:7C00H. Both UNIX and DOS use the command "fdisk"
51 [ 386bsd does not have an 'fdisk' (yet) ]
52 to install this first sector into the hard disk and to manipulate
53 the hard disk partitions.
57 2. The First Stage Bootstrap Loader
58 --------------------------------
60 After startup, the first stage boot is loaded at 0000:7C00H. This
61 first stage boot will load itself and the second stage boot into
62 memory at location 0000:1000H. For floppy disks, the first cylinder
63 is reserved as the boot cylinder, and the boot code (first and second)
64 will be loaded from there. Currently, only double sided, high density
65 (15 sectors per track) floppies are supported. For hard disks, the
66 first 29 sectors of the active partition is reserved for boot code
67 which will be loaded by the first stage boot. All the disk types
68 recognized by BIOS are supported by this bootstrap loader.
69 [for 386bsd we load the second stage booter to 9000:0]
73 3. The Second Stage Bootstrap Loader
74 --------------------------------
76 After the boot code is loaded, the control is passed to the second
77 stage bootstrap loader "boot2()". In order to be able to load the
78 big kernel image (bigger than 512K or 640K, depends on the memory
79 configuration), the second stage boot loader will run on the protected
80 mode. This bootstarp loader does not have any stand alone device
81 drivers, all the I/O's are through the BIOS calls. Since the first
82 stage boot code will no longer be used at this moment, the memory
83 location of the first stage boot code (0000:1000H to 0000:1200H) will
84 be used as an internal buffer for BIOS calls. Immediately after this
85 internal buffer is the GDT table for the second stage boot loader.
86 Since this boot loader needs to switch back and forth between protected
87 and real mode in order to use BIOS calls, the limit of the boot code
88 and boot data segments must not be greater than 64K.
90 The boot loader loads the kernel image at memory location above 1 MB
91 to skip the memory hole between 521K/640K and 1MB. After the kernel
92 is loaded, the boot loader stores the information in the stack and
93 then passes control to kernel. Currently, the three information passed
94 fromm the boot loader to the kernel are type of the boot device, size
95 of the base memory and size of the extended memory.
97 [ 386bsd receives: howto, bootdev]
99 [ 386bsd is loaded where-ever the "MByte" bits of the load address specify,
100 so if you link it for FE100000 it will load to 1MB, but if you link
101 it for FE000000 it will load ad 0MB]
103 [for machines with only 512KB normal ram the kernel will need to be linked
104 for 1MB and the bootblocks modified to run below 512KB. (8000:0)]
110 Since the boot loader loads the kernel image at memory location above
111 1MB, the kernel has to start as protected mode. In addition, the
112 link editor description file (vuifile) has to indicate that
113 the text and data segments start above 1MB. Also, the boot loader
114 passes the infomation to the kernel through the stack.
116 [MOST of what is mentionned below is NOT relevant to 386bsd]
118 5. Disk Layout and Bad Block Handling
119 ---------------------------------
121 The System V/386 Release 3.2 (AT) disk layout will be used as the disk
122 layout for the MACH System on the AT platform.
124 This disk layout is as follows:
126 * Reserve the first sector of cylinder 0 for the DOS boot record which
127 contains the master boot code (446 bytes) and the partition table.
128 (Refer to DOS Technical Reference Manual page 9-6 to 9-10).
130 * Reserve the first 29 sectors of the UNIX partition for the first
131 and the second stage bootstrap.
133 * Reserve the 30th sector of the UNIX partition for the pdinfo and
136 * Reserve the 31st to the 34th sectors of the UNIX partition for the
137 bad track and the bad block mapping tables.
139 * Reserve up to 253 consecutive tracks when required, beginning with
140 the 35th sector of the UNIX partition, for alternate tracks.
142 * Reserve up to 253 consecutive blocks, beginning with the first
143 sector after the alternate tracks area, for alternate blocks.
147 ----------------------------------------------------
149 ---------------- .......... --------------------
151 ---------------- .......... --------------------
153 ===============================================================
154 ^ | BOOTSTRAP | CYL N, TRK M
155 | ----------------------------------------------------
156 | | |30 |31 |32 |33 |34 |
157 ---------------------------------------------------- ---
159 N ---------------- .......... --------------------- |
160 I | .......... | Alternate Tracks
161 X ---------------- .......... --------------------- |
163 P ---------------------------------------------------- ---
165 R ---------------- .......... --------------------- |
166 T | .......... | Alternate Blocks
167 I ---------------- .......... -------------------- |
169 I ---------------------------------------------------- ---
170 O | Unix root partition starts from here |
171 N ---------------- -----------------
173 ----------------------------------------------------
175 ----------------------------------------------------
177 | ---------------------------------------------------
179 | ----------------------------------------------------
181 ===============================================================
183 --------------- ........ --------------
185 ----------------------------------------------------
188 The bad block handling mechanism is as follows:
190 * Use the alternate track in the alternate tracks area if the
191 track containing the target sector is bad.
193 * Use the alternate block in the alternate blocks area if the
194 target sector is bad.
202 Since the kernel image is loaded above 1 MB, the kernel must start
203 as protected mode. This means that this bootstrap loader will work
204 only when the corresponding changes on the kernel startup code are done.
206 The make command to generate this bootstrap loader is:
208 make -f boot.mk fdboot (floppy boot loader)
209 make -f boot.mk hdboot (wini boot loader)
210 [to make 386bsd bootblocks "make da wd" (warning: they will be installed
211 in /dev/mdec.. take backups)]